This application is based upon Japanese Patent Application No. 2000-398751 filed on Dec. 27, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) element, specifically to an aging treatment for restraining a luminance from changing in a manufacturing method.
2. Related Art
An organic EL element includes a luminescent layer containing an organic luminescent compound that is provided between a cathode and an anode. The electrodes (cathode, anode) and the luminescent layer are formed by sputtering or deposition.
Such organic EL element is self-luminous, thereby having a high visibility. Moreover, the organic EL element can be driven at low voltage such as several voltage to several ten voltage, so that reducing weight thereof including a drive circuit can be achieved. Therefore, the organic EL element is expected to be employed to a thin film type display panel, a lighting equipment, a backlight or the like.
However, it is difficult for the organic EL element to luminesce stably for a long time. To solve this problem, such a method has been adopted that the element is covered by something to be shielded from an ambient air, a durability of the element is improved, or the electrodes are driven at lower voltage.
As one of methods relating to extend a lifetime thereof, a method is known in which luminance is prevented from deteriorating with age so that element characteristics can be stabilized by conducting an aging treatment in which a current is applied between the cathode and the anode in the manufacturing process. The method is disclosed in JP-A-05-182764, JP-A-08-185979, JP-A-10-208880.
However, in the aging method in the prior art, the study for aging time is not performed sufficiently that how long the aging should be continued. Since the aging treatment is to be lowered luminance efficiency, when the treatment is conducted excessively, even if luminance is prevent from deteriorating with age, the luminance efficiency is lowered noticeably to cause a problem in which applied voltage increases.
Moreover, the aging method, disclosed in JP-A-08-185979 described above, is conducted at room temperature. However, this method require a long time such as approximately one day even if the aging is conducted at a condition in a current density of a hundred times as large as that in activating when the element is supposed to have a half-life of ten thousands hours at practical luminance (for example, several hundreds cd/cm2). This is a big problem in view of improving the efficiency in the manufacturing process of the element.
This invention has been conceived in view of the background as described above and an object of the invention is to provide a suitable aging treatment in manufacturing process of an organic EL element.
According to consideration by the inventors, a curve of change in luminance with time was measured and analyzed. Incidentally, the curve was obtained by measuring the luminance when the element is driven at constant current. The inventors found that the curve was composed of a plurality of components respectively different in luminance age-deterioration rate. The invention was made in this view.
According to a first aspect of the present invention, the curve of change in luminance with time is divided into a component of a slowest luminance age-deterioration rate and other component, and aging is conducted to such a degree that luminance of the element changes approximately equal to an initial value of the slowest luminance age-deterioration rate. By conducting such aging, in the curve, fast components of luminance age-deterioration rate in the curve are eliminated.
A change in luminance due to activation of the aged organic EL element decreases according to a deterioration rate of a component having slowest luminance age-deterioration rate, the organic EL element to be subjected to the aging treatment can be lengthen in lifetime thereof. Namely, the initial value of the slowest luminance age-deterioration rate is the best value for target luminance that indicates how long the aging should be continued.
According to the present invention, efficiency of aging time can be improved.
Preferably, when the aging is conducted, a change in luminance that is measured when the element is driven at a constant current is fitted to a curve determined by a predetermined formula.
According to another aspect of the present invention, a curve of change in luminance with time of luminance is obtained by measuring the luminance when the organic EL is driven at a constant current, and then analyzed. The curve of change in luminance with time is divided into a most dominant component in luminance age-deterioration degree and other component. Then, an aging is continued to such a degree that luminance deterioration corresponding to the most dominant component in luminance age-deterioration degree is approximately completed. By conducting this aging, in the curve, dominant components in luminance age-deterioration rate are substantially eliminated. Therefore, optimal aging can be conducted for restraining luminance of the aged element from deteriorating with age.
According to another aspect of the present invention, a curve of change in luminance with time of luminance is obtained by measuring the luminance when the organic EL is driven at a constant current, and then analyzed. The curve of change in luminance with time is divided into a component having luminance age-deterioration completed at shortest time, and other component. Then, an aging is continued to such a degree that luminance deterioration corresponding to the component having luminance age-deterioration completed at shortest time is approximately completed. By conducting this aging, in the curve, the component having luminance age-deterioration completed at shortest time is eliminated. Therefore, optimal aging can be conducted for restraining luminance of the aged element from deteriorating with age.
Preferably, the aging is conducted at a temperature between a room temperature and a temperature lower than a lowest glass-transition temperature of a material included in the organic EL element. This is because aging time can be shortened by conducting at the room temperature or more, and because the element deteriorates when the aging is conducted at higher temperature in comparison with the lowest glass transition temperature described above.
Preferably, current is applied to the element so that luminance in performing the aging exceeds practical luminance in the organic EL element to shorten the aging time.
Other features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings.